The invention relates to a cathode-ray tube comprising in an evacuated envelope means to generate at least one electron beam and to form a cross-over behind which, viewed in the direction of propagation of the electron beam, are provided successively an accelerating pre-focusing lens comprising a first and second lens electrode centred around an axis, and a main focusing lens comprising at least two electrodes. The cathode-ray tube includes means to apply electric voltages to the lens electrode.
Such a cathode ray tube can be used for displaying monochromatic or multicolour pictures, for example television pictures. In that case the cathode-ray tube is a display tube comprising a display screen. Such a cathode-ray tube, however, may also be used to record pictures. In that case the cathode-ray tube is a camera tube having a photosensitive, for example photoconductive, layer.
The means to generate an electron beam and to form a cross-over are usually formed by a triode consisting of a cathode, a control grid and an anode. However, it is also possible to generate an electron beam by means of a semiconductor device as described in Netherlands Patent Application No. 7 905 470, corresponding to U.S. Pat. No. 4,303,930, and to focus it to form a cross-over.
A cathode-ray tube of the kind described in the opening paragraph is known from the Netherlands Patent Application No. 7 902 868, corresponding to U.S. Pat. No. 4,318,027. The cathode-ray tube described in the latter Application comprises a cathode, a control grid, a first lens electrode (termed G-2 in the Application) and a second lens electrode (termed G-3 in the Patent Application). The cathode, control grid and first lens electrode ensure the generation of an electron beam which is focused into a cross-over in the proximity of the first lens electrode. The cross-over is focused onto the display screen of the cathode-ray tube by means of one or more focusing lenses. The spot displayed on the display screen must have a good quality. This means that the spot must have small dimensions and be surrounded by as little haze as possible. According to the published Netherlands Patent Application No. 7 902 868 a spot having a better quality is obtained by using a comparatively thick first lens electrode and by ensuring that a strong flat electric field is present between the first and second lens electrode and/or the main focusing lens has a larger object distance than usual. According to the Patent Application the pre-focusing of the electron beam by the first accelerating focusing lens is preferably eliminated or is at least strongly reduced after the cross-over. Nevertheless, the improvements described do not lead to an optimum result.